The overall objective of this project is to obtain detailed information on the role of exogeneous factors and intrinsic genetic program determining the differentiation of muscle fibers in embryonic, adult and aging muscles. To study in detail these various age-related changes and external factors we will investigate the regulatory proteins and the myosin molecule under physiological conditions (development and aging) or under conditions of an imposed change such as changed activity pattern by chronic continuous or intermittent nerve stimulation.